Evaluate predictions with nerval

Based on !340 (merged)

Closes #231 (closed)

configs/eval.json

@@ -19,7 +19,7 @@
                 ["training", "test"]
             ]
         },
-        "max_char_prediction": 30,
+        "max_char_prediction": 200,
         "tokens": "tests/data/prediction/tokens.yml"
     },
     "model": {